Pressurized mill and process

ABSTRACT

In a comminuting or dispersing mill a body of agitated grinding medium in a retaining case is pressurized, preferably by inflatable elastic means.

United States Patent 1191 Clark Dec. 23, 1975 PRESSURIZED MILL AND PROCESS 3,298,618 1/1967 Talpey 24 7462.1;

. 3,332,628 7/1967 Wadham 241 I7 [75] inventor: Leroy Clark Huron 3,539,117 11/1970 Sjogren 241/172 [73] Assignee: SCM Corporation, New York, NY. 1

[ Filed: July 5, 1974 Primary Examiner-Granville Y. Custer, Jr. [211 App}. 486,011 Attorney, Agent, or FirmJerry K. Mueller, Jr.

[52] US. Cl 24l/46.11; 241/172 [51] Int. Cl. B02C 17/18 [57] ABSTRACT [58] Field of Search 241/4611, 46.15, 46.17,

2 1/1 172 In a comminuting or dispersing mill a body of agitated grinding medinm in a retaining case is pressurized, [56] References Cited preferably by inflatable elastic means.

UNITED STATES PATENTS 3,243,128 3/1966 Tight 241/172 x 6 Claims, 1 Drawing Figure m FEED US. Patent Dec. 23, 1975 3,927,837

PRESSURIZED MILL AND PROCESS This invention relates to a comminutingor dispersing mill which contains a body of agitated grinding medium, and more particularly to flexibly pressurizing said body in said mill.

Exemplary prior mills to which this improvement is applicable are represented by the stirred media type (typified by US. Pat. Nos. 2,764,359 and 3,809,545)

and the type wherein the case retaining the media is vibrated or gyrated (typified by U.S. Pat. Nos. 2,982,485 and 2,983,454). The disclosures of these patents are expressly incorporated herein by reference. Such mills generally are used for wet and dry grinding of particles, dispersing solids in liquids such as pigments in paints, and mechanical alloying of metals. For convenience herein the term grinding will be used to comprehend grinding, dispersing, and otherwise milling particulate solids with conventional grinding media such as stones, metal balls, ceramic shapes and the like.

Advantages of this improvement over prior proposals include improved control of grinding of solids and/or dispersion of solids.

The instant improvement in such mill comprises: a case; a body of particulate grinding medium retained in said case; an agitator for said medium; and pressurizing means for flexibly pressurizing said body of medium in said case.

The instant improvement in milling process wherein solid particulate grinding medium is agitated in a milling zone comprises: flexibly pressurizing said medium for restricting its freedom of movement in said zone.

The accompanying drawing shows in cross-sectional elevation a preferred mill used to grind paint according to the precepts of this invention. It will be more particularly described in the example which follows.

The mill case, grinding medium, and agitator therefor are like those conventional for the exemplary prior mills referred to above. Frequently and preferably the case is of metal or lined metal. If lined withelastomer, the resulting resiliency can even help to control pressure on the medium in accordance with this invention. The mill can be constructed conventionally to discharge against superor subatmospheric pressure, but generally atmospheric pressure discharging is favored for efficiency and economy. The case can be heated or cooled conventionally, if necessary or desirable.

Preferably the grinding medium is substantially all of the same kind, with approximately the same size and shape. Preferred shapes are broadly cylindrical or spherical, and they can be made of metal, stone, ceramic, metal carbide, glass, or like material conventional for grinding or dispersing. For grinding paint 5/32 to 9/16 inch balls are preferred, the undersized or unevenly worn shapes being discarded from time to time.

Preferably the agitator is internal, e.g., a driven stirrer like that shown in the drawing. Alternatively, it can be external, i.e. operating to vibrate or gyrate the case substantially full of grinding medium in conventional fashion. Internal agitators preferably are of metal for strength and they can be coated with wear-resisting material such as rubber or hard-facing alloy or cermet.

The preferred pressurizing means or member is an inflatable and deflatable elastic bladder positioned freely (not attached to other parts) in the case at or near the top for most delicate control and easy replacement. In typical priormills referred to above, the bladder can be retained within the mill case advantageously by, for example, a conventional woven wire screen or the like. Advantageously the bladder is annular in shape to fit around a central agitator of the preferred internally stirred mill. Inflation and deflation of such bladder (e.g. an automotive tire inner tube) can be done manually or automatically according to experience or in response to various indicia such as degree of grind on output, temperature of contents or output, viscosity of a slurry output, torque on an internal stirrer, loss of grinding medium, etc.

Alternatively the elastic pressurizing member can be attached to an inner part of the mill, advantageously a wall, top, bottom or other stationary part. The pressurizing also can be accomplished by dispersing in the grinding medium or concentrating in one place in the mill one or a plurality of deformable bodies, e.g. hollow like tennis balls, full like sponge rubber balls with or without molded skins, a flexible bag or bags stuffed with pieces of foamed elastomer, or the like. Adjustable pressure can be exerted on such deformable bodies by pressing them or the grinding medium around them more or less by, for example, screwing up and down on a screen retaining such bodies or medium in the mill, or displacing them more or less with a piston or the like, but this can be awkward in comparison to inflating and deflating an internal elastic member. While the pressurizing need not be adjustable during mill operation, it is often advantageous to have it so adjustable, e.g. between A and 20 and preferably 2-12 pounds per square inch of pneumatic pressure in a bladder. Thus the flexible pressurizing means is TAC- TILE i.e. a flexible surface actually pressing on some of the grinding medium as distinguished from liquid or gas back pressure in the mill case.

The present invention will be better understood by reference to the following example which is provided by way of illustration and should not be construed as in limitation. In this specification all temperatures are in degrees Fahrenheit, and all parts and percentages are by weight unless otherwise specifically noted.

EXAMPLE Referring to the drawing, the steel mill case 2, steel agitator 4 with arms 6, and motor drive (not shown) was a Szegvari Model No. C-3 pilot-size continuous attritor approximately 3 feet in height overall, having top outside diameter of about 9 inches and a bottom outside diameter of about 8 inches. There were 10 stirrer arms (items 6) along the length of shaft 4, alternately at right angles to adjacent arms and graduated in projection from 1 inch near the bottom of shaft 4 to 3 inches near the top of said shaft. Reinforced woven wire screen 9 was removably attached near the top of case 2 below the level of outlet wier ll. Woven wire screen 8 was positioned at the juncture of case 2 and feed inlet 7. The interior of case 2, except for the very top space occupied by inner tube 12, was randomly and loosely packed with alumina grinding bodies, having density about 4.5 and nominally 3/16 inch diameter but slightly lemon-shaped.

A rubber automotive inner tube about 9 inches in outside diameter and about 6 inches high when inflated was positioned between screen 9 and the top of the body of grinding medium 3. The tube was inflated and deflated through a valve projecting from a stem using nitrogen gas entering line 13 from a source not shown.

3 Inner tube pressure was maintained at a substantially constant value during a run by an automatic regulator not shown.

Pigment paste was made by continuously pumping a 4 was smoother than a corresponding paint using conventional pebble mill pigment grind.

I claim: 1. In a millingprocess wherein a body of solid particslurry of mineral pigment and vehicle mixture into inlet ulate grinding medium is agitated in a milling zone, the 7 by a pump, not shown. Finished ground paste continimprovement which comprises: pressurizing a surface uously overflowed wier 11 at the top of case 2. The of said body of particulate grinding medium by elastic mineral pigment used was a mixture of pigmentary inflatable means retained within said zone in contact rutile titania, talc, clay, barium sulfate, alumina, iron with said surface, said pressurizing means transferring oxide, and chromium oxide. The vehicle mixture was substantially uniform pressure to said surface in water containing minor amounts of conventional surcontact therewith. factants, ethylene glycol, and a fungistat. 2. The process of claim 1 wherein said pressurizing is A series of test runs were made using various inner controlled by inflating and deflating said elastic memtube inflation pressures and shaft speeds. The results ber situated in said zone. from this series are tabulated below. 3,. The process of claim 1 wherein said pressurizing TABLE Average Slurry Inner Feed Residence Output Output Mill Tube Pump Time l-lagmann Paste Shaft Pressure Pressure in Mill Guage Visc. Run (RPM) (PSIG) (PSIG) (Min.) Grind Krebs Units 1 160 0 3 15 195-2 93 (Control) 2 I60 2% 5 l5 3 105 3 140 5 7 l5 3 l l l 4 140 7% 8 15 3 not determined 5 l l0 10 l I I5 3 143 From these runs and others where mechanical problems caused their discontinuance it was concluded that inner tube pressures between about 2 and 12 psig were preferable for best operation with the lower pressures being favored for their less wear on the grinding medium. When the inflation pressure was psig, no noticeable improvement in degree of grind was obtained. However, modest pressures above atmospheric gave pronounced improvement in degree of grind and increase in paste viscosity. When inflation pressure exceeded psig, stirring became quite difficult without appreciably improving pigment grind but substantially increasing grinding medium wear.

When the thus super-pressure ground paste was incorporated into a conventional latex paint, the paint appeared to have greater hiding power, more gloss and means is pneumatically inflated to between about 1k and 20 psig.

4. A mill comprising: a case, a body of particulate grinding medium retained in said case; an agitator for said medium; and elastic inflatable pressurizing means retained within said case in contact with a surface of said body of particulate grinding medium, said pressurizing means transferring substantially uniform pressure to said surface in contact therewith.

5. The mill of claim 3 wherein said agitator is a stirrer for said medium, and said pressurizing means is disposed over said body of medium and around said stirrer.

6. The mill of claim 5 wherein said pressurizing means is pneumatically inflated to between about A and 20 psig.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,927,837

DATED I December 23, 1975 INVENTOR(5) I Leroy P. Clark It is certified that error appears in the above-identified patent and that said Letters Patent 6 are hereby corrected as shown below:

Claim 5, line 39. ang "3" to LP q ngncd and Scaled this sixteenth Day of March 1976 [SEAL] Attest: d

RUTH C. MfiSON C. MARSHALL DANN Atlestmg Ojjrcer Commissioner oflatents and Trademarks 

1. In a milling process wherein a body of solid particulate grinding medium is agitated in a milling zone, the improvement which comprises: pressurizing a surface of said body of particulate grinding medium by elastic inflatable means retained within said zone in contact with said surface, said pressurizing means transferring substantially uniform pressure to said surface in contact therewith.
 2. The process of claim 1 wherein said pressurizing is controlled by inflating and deflating said elastic member situated in said zone.
 3. The process of claim 1 wherein said pressurizing means is pneumatically inflated to between about 1/2 and 20 psig.
 4. A mill comprising: a case, a body of particulate grinding medium retained in said case; an agitator for said medium; and elastic inflatable pressurizing means retained within said case in contact with a surface of said body of particulate grinding medium, said pressurizing means transferring substantially uniform pressure to said surface in contact therewith.
 5. The mill of claim 3 wherein said agitator is a stirrer for said medium, and said pressurizing means is disposed over said body of medium and around said stirrer.
 6. The mill of claim 5 wherein said pressurizing means is pneumatically inflated to between about 1/2 and 20 psig. 